Along with sharp increase of data traffic in mobile Internet, each operator explores use of a Long Term Evolution (LTE) technology on an unlicensed band, thereby developing a technology named License-Assisted Access (LAA).
An unlicensed band is open for all operators, and different operators have the same right of deploying LTE on the unlicensed band. For enabling different systems or different operators to fairly compete for using the unlicensed band, a Listen Before Talk (LBT) mechanism is introduced into an LAA technology. That is, a period of time is reserved at first to sense a carrier before data transmission of each time. Data transmission is started only when the carrier is sensed to be available. Moreover, there is a maximum time length limit made to data transmission of each time. Thus, it can be seen that the LBT mechanism is introduced into LAA just for avoiding a conflict between different systems or different operators in use of the unlicensed band and may not actually implement coordinated use of the unlicensed band between different systems and different operators.
In an LTE system, a Hybrid Automatic Repeat reQuest (HARQ) mechanism is usually adopted to ensure Quality of Service (QoS). In the LAA, a Carrier Aggregation (CA) architecture is mainly considered to be used. In a related CA architecture, HARQ retransmission and initial transmission are located on the same carrier.
Since an unlicensed band is open for all operators and all wireless access systems and different operators or different systems may not coordinate with each other, it is impossible to ensure that a carrier may be immediately adopted for data retransmission after a failure of data transmission on the carrier of the unlicensed band, which causes a relatively long HARQ delay. If the carrier is still unavailable after a relatively long time, data may finally be dropped, thereby reducing network performance.
Thus, it can be seen that a related HARQ mechanism may cause a relatively long data retransmission delay and even a packet loss phenomenon, thereby reducing network performance.